ICMCTF1998 Session B1-3: Hard Coatings - Evaporation & Arc Technologies

The objective of the present work was to determine the influence of positive bias on plasma and macroparticle (MP) flow in curved magnetized plasma duct. The plasma bulk and sheath regions were analyzed. In the plasma bulk the current density and electrical field component normal to the wall were obtained and used as boundary conditions for the near wall sheath region. In the sheath a non-stationary model for MP charging and motion was developed.

MP trapping in the near wall sheath was found. For example, titanium MP's with a radius less than 0.4 μm and with a velocity component normal to the wall of about 10 m/s are trapped if the sheath potential drop exceeds 30 V. MP's may move in the sheath region along the wall by a repetitive process of electrostatic attraction to the wall, mechanical reflection and neutralization, followed by MP charging and attraction, etc. The effective sticking coefficient of MP to the wall increases due to increasing number of MP collision with a wall by factor of 10⁴. Using an experimental MP size distribution it was obtained that the MP transmission fraction through filter decreases down zero due to the trapping effect when a bias potential of +80 V is applied between the wall and the plasma.

The following paper is an overview of the factors affecting the defect growth (DG) in Cathodic Arc deposited coatings. It is a common knowledge that the macroparticles contribute to the coating structure, but the exact mechanism has not been extensively discussed in the literature.

The first part of the paper deals with the issue of how defect growth is affected by DC bias voltage, frequency and duty cycle of unipolar pulsed bias voltage, and substrate temperature during deposition. It will be shown that : (1) Increasing the DC bias voltage will significantly suppress the DG; (2) Frequency of the pulsed voltage in a range of 0-33.3 KHz has no significant effect on the DG; (3) Decreasing the duty cycle significantly promotes the DG; and (4) Lowering of the substrate temperature results in an increase of the DG. A mechanism for the observed phenomena is suggested.

The second part of the paper deals with conventional means of minimizing the defect growth via shortening the metal ion bombardment by using alternative means of substrate heating and cleaning, I.e., radiant heating, glow discharge, electron heating, etc.

In the third part, emerging industrial solutions of suppression of the macroparticle emissions are described. One of the methods describes a Ti mushroom like cathode, on which an arc discharge is sustained at currents of 100-250 amp in nitrogen partial pressure. As a result of the cathode shape the latter heats up and a deep nitrided layer is produced. The resultant coatings have significantly fewer defects.

Another method called RGCA (Reactive Gas-Controlled Arc) has been reported to have produced similar results by formation of a deep nitride layer on the cathode surface. Here too, the deposited coatings exhibit high quality and significantly low defect concentration.

A triple-cathode vacuum arc plasma gun was used to deposit Ti-Zr-N and Ti-Nb-N multicomponent coatings onto cemented carbide (90%WC, 1.8%TaC, 0.2%NbC, and 8%Co) substrates. The coatings were deposited at a bias voltage of -40 V relative to the anode, and a substrate temperature of 400 °C, and at various nitrogen pressures ranging from 5 to 15 mTorr. The analytical methods used were SEM, XRD, AES, and the microhardness was measured by Vickers indentation.

It was shown that for the Ti-Zr-N coatings a solid solution (Ti,Zr)N was formed, in which the elements Ti, Zr, and N were distributed homogeneously. The (Ti,Zr)N grains grew in the (111) orientation. The nitrogen concentration in the solid solution was not affected by the nitrogen background pressure. However, with increasing the nitrogen pressure to 15 mTorr the Ti concentration increased, while that of Zr decreased, and a finer (nanocrystalline) structure was formed. The formation of a (Ti,Nb)N solid solution was observed in the Ti-Nb-N coatings. However, with increasing the nitrogen pressure to 15 mTorr traces of δ-NbN were also identified. The formation of the two phases is explained by a higher flux of Nb ions as compared to that of Ti. The correlation between the structure, the composition, the macroparticle content, and the microhardness will be also presented and discussed.

Thin metal/ceramic coatings were deposited onto polysulfone S2010 substrates using a triple-cathode vacuum arc plasma source connected to a magnetized plasma duct in order to improve the tribological properties of the surface. Various combinations of multi-layer coatings having Ti, Zr, or Nb sub-layers, and their nitrides as wear-resistant layers, were deposited and evaluated.

The deposition parameters (arc current, magnetic field strengths, deposition time) were optimized (1) to obtain necessary deposition rate and coating thickness, while preventing substrate damage under the high-energy ion flux exposure, and (2) to obtain good adhesion of the coating to the substrate at low substrate temperatures.

The structure and composition of the coatings were studied using XRD, AES, and SEM. Scratch tests were used to evaluate the adhesive strength between the substrate and the coating, and reciprocating wear tests against a steel ball were used to study the friction and wear rates of the coated samples.

It was shown that nitride layers possessed a nanocrystalline structure or a mixture of an amorphous and a nanocrystalline structure with random orientation. The lowest wear rate was observed for a Ti/TiN bi-layer coatings. It was observed that TiC formed at the interface of the Ti coating and the substrate. In contrast, Zr intermediate layers did not form a carbide, and coatings had poor wear resistance. The results suggest that the formation of a carbide interface improves the coating adhesion. The influence of coating composition on its adhesion to the substrate, interface formation, and the tribological properties will be presented and discussed.